The present invention relates to phosphors applicable to, for example, white-light-emitting diodes, semiconductor light emitting devices on which the phosphors are integrated, and fabrication methods thereof.
GaN-based Group III-V nitride semiconductor (InGaAlN) has a wide band gap (for example, GaN has a band gap of 3.4 eV at room temperature), and thus serves as a material that can realize a high-power light emitting diode in the wavelengths ranging from green and blue to ultraviolet. Blue and green-light-emitting diodes have already been in practical use in various displays, large-screen displays or signals. Furthermore, a white-light-emitting diode that can emit white light by exciting a YAG phosphor using a blue-light-emitting diode is being actively researched and developed to achieve higher brightness and an improvement in luminous efficiency, aiming at the implementation of semiconductor lighting which replaces current fluorescent lamps, incandescent electric lamps or the like. In particular, a white-light-emitting diode for lighting is expected to create a big market. In lighting application, in addition to improvements in brightness and luminous efficiency, it is important to improve the way a color is viewed, i.e., color rendering properties, when the diode is used for lighting.
Until now, a white-light-emitting diode, which emits white light by exciting a YAG phosphor using light with a wavelength of about 470 nm emitted from a GaN-based blue-light-emitting diode to obtain yellow light emission, has been developed and commercialized.
However, in such a white-light-emitting diode, the component of red light emission in emission spectrum is small, and therefore, the white-light-emitting diode causes a problem since it has poorer color rendering properties compared with a conventional fluorescent lamp or incandescent electric lamp. In this case, if excitation light can have a wavelength as short as that of ultraviolet light, a phosphor material used in a typical fluorescent lamp or the like can be utilized. However, in order to shorten the emission wavelength of a GaN-based light emitting diode, it is necessary to use AlGaN with a high Al composition which has difficulty in providing crystal growth. In addition, since the brightness or lifetime largely depends on the density of crystal defects, it is generally difficult to realize a GaN-based light emitting diode that can emit light with a wavelength as short as that of ultraviolet light. Besides, an ultraviolet-light-emitting diode, which can emit light with a wavelength of 350 nm or less, reportedly has an emission output of only about a few mW, and therefore, such a diode needs to be further improved in the future.
Under the circumstances, in order to improve the color rendering properties in white light emission, it is necessary to increase the brightness of a phosphor material that emits red light, in particular; furthermore, in consideration of the situation in which it is difficult to shorten the wavelength of light, there is a demand for a phosphor material that can realize strong red light emission by excitation light with a longer wavelength.
Hereinafter, with reference to FIG. 16, description will be made about a conventional white-light-emitting diode that realizes white light emission by exciting a YAG phosphor using a GaN-based blue-light-emitting diode as already mentioned above.
FIG. 16 is a cross-sectional view illustrating the structure of a conventional white-light-emitting diode that uses GaN-based semiconductor.
As shown in FIG. 16, an n-type InGaAlN layer 101, an active layer 102 made of InGaAlN, and a p-type InGaAlN layer 103 are formed in this order from below over a sapphire substrate 100 by an MOCVD process, for example. In this structure, the active layer 102 emits blue light having a wavelength of 470 nm when electric current is applied. On a surface of the n-type InGaAlN layer 101 from which a portion thereof is etched away for electrode formation (using a Cl2 gas, for example), an electrode 105 made of Ti/Au is formed, and a transparent electrode 104 made of Ni/Au is formed on the p-type InGaAlN layer 103 that has been subjected to the etching. On the transparent electrode 104, an electrode 106 made of Au for bonding pad formation on the side of the n-type InGaAlN layer 103 is selectively formed. Thus, with the use of the transparent electrode 104, most of blue light emitted from the active layer 102 is transmitted through the transparent electrode 104 and taken out outside. It should be noted that in order to allow the transparent electrode 104 to serve as a transparent electrode, its thickness has to be 10 nm or less.
After a wafer functioning as a blue-light-emitting diode having the above-described structure has been divided into light emitting diode chips each having a size of, for example, 300 μm×300 μm, each chip is mounted on a package surface 107, and wire bonding is carried out. Thereafter, a material of a YAG phosphor 108 is dropped over a package and is hardened. Thus, the white-light-emitting diode having the structure shown in FIG. 16 is realized (see, for example, Japanese Unexamined Patent Publication No. 2002-246651). It should be noted that as the other white-light-emitting diode, a conventional example disclosed in, for example, Japanese Unexamined Patent Publication No. 2001-257379 is known.
Meanwhile, the above-described conventional white-light-emitting diode exhibits a luminous characteristic shown in FIG. 17. Specifically, as shown in FIG. 17, the conventional white-light-emitting diode realizes white light emission by mixing blue light emitted from a blue-light-emitting diode and yellow light emitted from a YAG phosphor. However, as apparent from FIG. 17, since the component of red light emission in emission spectrum of the white-light-emitting diode is small, it presents a problem that only white light emission poor in color rendering properties is obtained.